Defining and Locating the Seaward Landfast Ice Edge in Northern Alaska

The seaward landfast ice edge (SLIE) is simple to define conceptually but more difficult to determine in practice, partly since both the location and a lack of motion must be determined over time, implying that a single observation is insufficient. We examine different definitions of landfast ice used by others and then propose our own, consisting of two criteria: 1) the ice is contiguous with the land, and 2) the ice lacks detectable motion for approximately 20 days. This definition is applied to a time series of Radarsat synthetic aperture radar (SAR) imagery of the northern Alaska coast. Analysis reveals the presence of nodes where the SLIE exists more frequently. Nodes occur near the 20m isobath, suggesting grounding helps stabilize the SLIE in these locations. Comparison with studies from the 1970’s suggests that the stable extent of landfast sea ice has changed very little. Further work will carried out to determine whether there has been any change in the annual cycle. We also note that any comparisons must take into account relevant definitions and methods. This is particularly so with operational datasets that lack the benefit of hindsight such as the National Ice Center Ice Charts, which can differ significantly from our own for the same time period.